1. Field of the Invention
The subject invention relates generally to load cells and, more particularly, to electronic circuitry for improving the electrical output of such load cells.
2. Description of Related Art
In the prior art, load cell circuitry is known wherein strain gauges are employed in a Wheatstone bridge circuit. The bridge is subject to excitation voltage and produces an output signal. While providing useful measurements in various applications, such as shear beam (single and dual bridge) load cells, column load cells, cantilever beam load cells, and platform load cells, such load cells are nevertheless subject to various inaccuracies in their output signals.
Inaccuracies in load cell outputs may arise from nonlinearity, creep, hysteresis, and modulus compensation error. "Nonlinearity" refers to the departure in the signal output voltage from a straight line from zero load through a range of measured force. Typically, a calibration curve is developed, which is a record of the comparison of load cell output against a range of standard test loads from a no load state up to the so-called "rated load," which is the maximum load (force) the load cell is designed to measure within its specifications. "Nonlinearity" then refers to the deviation of the calibration curve from a straight line drawn between the no load and rated load output.
"Modulus compensation error" refers to a variation of the load cell signal output with temperature. Such errors may be either positive or negative going.
Load cell "creep" arises because the steel or other metal to which strain gauges are attached in a load cell stretches over time when a force is applied to the load cell. Creep results in an increasing or decreasing, rather than level, output signal from the load cell, when all other variables, including the weight being measured, are constant. Strain gauges used in load cells typically have a built-in negative creep to compensate for the positive creep of the load cell metal. The amount of creep typically varies from load cell to load cell due to manufacturing variances. Hence, the built-in creep compensation is not entirely effective.